Shell and tube exchangers of the general type to which this invention relates are well known. For a detailed description and explanation of the mode of operation of such heat exchangers, reference may be made to standard texts such as George Brown and Associates, UNIT OPERATIONS, 1953, John Wiley and Sons Inc. Such heat exchangers are used in many commercial processes where the heat exchange of fluids is necessary. For example, heat exchangers are used in the manufacture of ammonia gas wherein the synthesized ammonia exits from the ammonia reactor at a temperature of approximately 950.degree. F and is cooled by passing it through a heat exchanger. One example of a process of producing ammonia in which such heat exchangers are used, is U.S. Pat. No. 3,442,613 to Grotz.
In shell and tube heat exchangers for high pressure service, a hot gas is typically cooled by passage through the tube bundle in heat exchange relationship with steam or water which is on the shell side. The hot gas decreases significantly in temperature and part of the water is vaporized to produce steam. Accordingly, such an exchanger may be used not only to cool the hot gas but also as a steam generator.
A problem associated with heat exchangers and steam generators of this type has been the high thermal stresses caused by the large temperature differences through the tube sheet of the exchanger and between the inlet and outlet sides of the tube sheet on the front face where it is separated by the passrib or pass partition. For example, where inlet water in a steam generator is at a relatively low temperature and the water is passed in heat exchange relationship with the hot gas, the tube sheet is subjected to very high thermal stresses due to the large temperature difference between the hot and cold sides of the tube sheet and in the region of the pass partition which separates the hot inlet synthesis gas from the cooled outlet side. These temperature differences may be on the order of as much as 275.degree. to 330.degree. F. It is now recognized that such large thermal stresses can cause a number of problems in a heat exchanger and may even eventually result in failure or leakage of the exchanger.
Another problem which as been found in shell and tube heat exchangers previously utilized is that of crevice corrosion which may occur on the back face of the tube sheet. A small gap exists between the outside diameter of the tubes of the tube bundle and the inside diameter of the openings in the tube sheet through which the tubes extend. In many geographical areas, the feed water which is used to cool the hot effluent contains chlorides, and when the exchanger is shut down for maintenance or for any other purpose, the steam condenses and deposits chlorides in such small gaps which attack the tubes and tube sheet and cause corrosion to occur.